The invention relates to a method for manufacturing compound material wafers. The invention is particularly applicable to fabricating semiconductor on quartz type compound material wafers.
The SMART-CUT® technique can be modified to fabricate compound material wafers, such as heterostructures. It is necessary to modify the SMART-CUT® technique to account for stresses inside the compound material wafers, as the heterostructures may have different physical properties. For example, the heterostructures may have different thermal expansion coefficients. These stresses can lead to bad quality wafers, or may even result in the complete destruction of samples during processing.
The SMART-CUT® technique includes three main process steps which are:    a) forming a predetermined weakened zone in a source substrate, b) attaching the source substrate to a handle substrate to form a source-handle assembly, and c) detaching the predetermined weakened zone by providing energy, in particular by applying thermal energy.
Since compound material wafers are composed of materials that usually have different physical properties, like different thermal expansion coefficients, a thermal annealing step leading to detachment cannot always be completely conducted because to do so would lead to excessive stress inside the source-handle assembly. One proposed solution, described for example in published International Application No. WO 01/11667, is to separate the weakening and detachment step into two parts. The first part consists of a thermal treatment to weaken the predetermined weakened zone, and the second part concerns physically detaching the source substrate at the predetermined weakened zone from the source-handle assembly such that a thin film is transferred from the source substrate onto the handle substrate. During the weakening step the predetermined weakened zone becomes weaker and weaker. However, the thermal budget, which is a combination of temperature and time, necessary to complete the detachment is not completely applied so as to prevent detrimental consequences due to the presence of high stresses inside the source-handle assembly. After the weakening step, the source-handle assembly is cooled down to room temperature to carry out the final detachment step. Additional energy is then provided, typically mechanical energy, to achieve the physical detachment. A blade is usually introduced at the weakened plane to supply the necessary energy.
This proposed process makes it possible to create compound material wafers, even if different thermal expansion coefficients are present. However, in the context of a mass production process, it has been observed that the quality of the final product is not always good enough which results in unsatisfactory production yields and thus an increase in costs per good quality wafer. During mass production, wafers are typically thermally treated in groups of 25 or more, wherein 100 wafers is a common amount. Thereafter, the source substrate of each wafer has to be mechanically detached using detachment equipment, such as a blade, at room temperature. The furnace is then free for further utilization. It has been observed that this type of processing increases the roughness of the wafer. Moreover, the transferred layer does not fully cover the surface of the handle substrate, and the free zone, called the exclusion zone, is large. Furthermore, the rim of the transferred layer has large irregularities at its circumference. All of these characteristics result in a reduced quality product.